1. Field of the Invention
This invention relates to a condenser and a method of operating a main steam condenser of a steam turbine driven by steam from a boiler, with condensate in the main condenser being fed to the boiler.
2. Description of the Prior Art
A main steam condenser having an isolatable condenser hot well unit is proposed in JP-A-2-95704 and is illustrated in FIG. 8 of the drawings of this application. The condenser includes a tube bundle 28 in a tube bundle unit 2 comprising condensing tubes for cooling and condensing the steam turbine exhaust steam with sea water or the like, and a hot well unit 3 for storing the condensate, isolated by a condenser partition 7 having a shut-off valve 4. The shut-off valve 4, disposed in the partition 7, is open in normal running so that the water condensed in the tube bundle unit 2 is stored in the hot well unit until it is fed to the boiler. When the operation of the condenser is interrupted, the shut-off valve 4 is closed to leave only the tube bundle unit 2 open to the atmosphere so as to prevent the condensate in the hot well unit from deteriorating by oxygen or the like dissolved in the condensate during the interruption of the operation of the condenser. At start-up of the plant, the drainage stored in the tube bundle unit during the interruption is discharged to outside via a drainage pipe 9 having a valve 10 and vacuum in the tube bundle unit 2 is raised by an air extractor 17 in a line 18. The shut-off valve 4 is opened to connect the tube bundle unit 2 and the hot well unit 3 when their pressures are substantially equal to each other.
The partition-type condenser of this construction is being widely adopted in a combined cycle plant for daily start-stop (DSS) operation in which the condenser is started and stopped every day so that it may be used a daytime power source. By the start-up method described and the condenser used therefor, the plant can be started for a short time period and with power economy.
As mentioned, the drainage accumulated on the condenser partition 7 while the condenser operation is interrupted can be discharged to the outside of the condenser, but no consideration is made for the treatment of the drainage which is generated at the start-up to the plant. Specifically, the drainage generated during the period or interruption of operation when the condenser is open to the atmosphere contains a considerable amount of oxygen, so that the drainage cannot be mixed with the condensate in the hot well unit. Therefore, the drainage which is accumulated on the condenser partition during the stop period of the condenser is discharged to the outside of the condenser. On the other hand, the drainage generated at the initial stage of the plant start-up has low quality, but in a later period has a high quality. Thus, it is effective for economical plant operation to recover those two types of drainage to the condenser, but this has not been considered in the prior art.
Incidentally, according to the prior art, the drainage accumulated in the condenser tube bundle unit 2 at the start may be mixed directly into the hot well unit 3 by opening the shut-off valve at an early stage after the start. However, since the drainage at this stage contains considerable amount of oxygen, the quality of the condensate water after the mixing highly departs from the required value for the boiler supply water so that hot well condensate 29 has to be de-aerated.
It is also conceivable to recover the drainage during the start-up to the outside of the condenser. However, discharging drainage in this way while raising the vacuum requires a powered drainage discharger for discharging it to atmospheric pressure, because the pressure of a tube bundle unit 2 of the condenser 1 is negative. If such a drainage discharger is used, ambient air may flow back into the condenser 1 to obstruct or delay the rise of the vacuum in the condenser.
A further matter which is given little consideration in the prior art is recovery of the condensate of turbine gland seal steam. When a turbine has a steam gland, the gland seal steam discharged from the steam gland may be condensed in a gland steam condenser 15. Steam is fed to this condenser 15 via line 21. The heat exchange is effected in condenser 15 by main steam condensate from the hot well 3, which may be recirculated directly to the hot well via line 16. The destination of gland steam condensate from the condenser 15 is not mentioned.
JP-A-4-112903 proposes a turbine steam condenser system in which the gland steam condensate is returned directly into the hot well. There is no partition in this case between the hot well and the tube bundle unit and the proposed method is disadvantageous in that it is now appreciated that initially at start-up of the turbine, the gland steam has a high oxygen content, so that its condensate is unsuitable to be fed into the main steam condensate, for re-feeding to the boiler.
U.S. Pat. No. 5,095,706 proposes a partition between-the hot well and the tube bundle unit similar to JP-A-2-95704.